There's a couple of riverside festivals going on around town this weekend. In the District Sunday morning at 9, day-long waterfront festivities get under way with a backstage tour of the Arena Stage complex, Sixth and Maine SW, and at 10, on the theater's south lawn, a flea market that's boasting an assortment of notions plus props, used costumes and furnishings from past productions. More than 300 pleasure craft, commercial boats, police, fire and Coast Guard ships will sail up the river from the Wilson Bridge at 11 in a waterborne parade led by the 195-foot self-propelled barge, Point Counterpoint II, -- the floating home of the American Wind Symphony Orchestra. The vessel will be anchored at the Police Pier, 550 Water Street, through June 10 for tours. At noon, the Southwest Waterfront festival starts with free entertainment on three stages, crafts workers, exotic foods and a fashion show at 1:30 at the Gangplank Marina. The 45-member orchestra will give a free concert Wednesday night at 8, climaxing with an explosive fireworks display over the Washington Channel. Call 554-9066 for more information on the barge and festival. Across the river, Old Town is paying homage to its shore with two days of waterfront celebrations. The Canadian schooner Bluenose II will be docked along the north pier, directly behind the Torpedo Factory, with a Viking ship, the Skipjack Norfolk and other yachts. There'll be continuous live entertainment on the waterfront festival sites, pony rides, a moon bounce, food tents and a fireworks display Saturday night at 9:30. On Sunday morning at 9 there's a 10K race that starts at Founders Park on Union Street. The festival is sponsored by the Alexandria Red Cross. Hours are 10 to 10 Saturday and 9 to 5:30 on Sunday. Parking is limited; try carpooling or other public transportation to get there. Call 549-7268; free.